Glass fiber strands are typically formed by attenuating glass filaments through bushing tips or orifices at the bottom of a heated bushing containing molten glass. The filaments are then passed across the application surface of an applicator where they are coated with a binder and/or size. The filaments are then passed within the groove of a gathering shoe, which is typically a grooved cylinder or wheel formed of a material such as graphite, where the filaments are combined into one or more unified strands. The strands are then collected on a rotating drum or collet as a forming package.
In the past, it has been found advantageous to form the filaments and strand at one forming level and to collect the thus formed strand on a second forming level. This double-level operation has improved the quality of strand produced.
However, a major problem in the formation of quality glass strands remains in controlling the environment at and directly below the bushing. It is well known that as the filaments are attenuated through the bushing at high speeds, typically ranging from about 2,000 to 20,000 feet per minute (609.6 to 6096 meters per minute), that air is aspirated downwardly with the filaments at high speed.
The majority of the air aspirated downwardly passes through an opening in the floor separating the filament and strand formation level from the strand collection level. However, some of the air strikes the floor separating these levels. The air striking the floor has, in the past, had no place to go but to return towards the bushing. Behind the applicator is a metal deflector shield which runs from slightly above the applicator to the floor to help direct the air downwardly and to thus help prevent turbulent flow. On either side of the forming position are side shields to separate one forming position from the other. At the front of the forming position is a small region in which the operator may work, however, this region has a wall enclosure at its front. Thus, the only direction which the air striking the floor of the strand formation level to go is to return towards the bushing.
The return of this air causes turbulent air flow in the region directly below the bushing. Turbulent air flow is non-uniform air flow and leads to non-uniform temperature conditions directly below the bushing. Since filament diameters are directly proportional to glass viscosity for a given bushing tip size and since glass viscosity is directly proportional to the temperature of the glass at each orifice, variations in temperature in the region directly below the bushing produces uneven diameter strands. If severe enough, the temperature conditions may even produce filament breakouts. In addition, if the air turbulence is severe enough, the turbulence alone may cause filament breakouts.
Thus, it is desirable to maintain a uniform air flow and a uniform air temperature environment in the region directly below the bushing.